feat: add packaging, deployment, error pages, and project docs

Phase 7 — Polish:
- Error page template with styled 404/403/500 pages
- Error rendering helper on Renderer

Phase 8 — Packaging & Deployment:
- Containerfile: multi-stage build, non-root user, health check,
  OCI labels with build date and git revision
- Makefile: build, test, cross-compile, deb, rpm, container,
  tarballs, checksums targets
- nfpm.yaml: .deb and .rpm package config
- systemd service: hardened with NoNewPrivileges, ProtectSystem,
  ProtectHome, PrivateTmp, RestrictSUIDSGID
- Default environment file with commented examples
- postinstall/preremove scripts (shellcheck validated)
- compose.yaml: example Podman/Docker Compose
- Caddyfile.example: subdomain, subpath, and remote proxy configs
- CHANGELOG.md for release notes
- CLAUDE.md with architecture, conventions, and quick reference

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dist/favoritter.service

@@ -0,0 +1,25 @@
+[Unit]
+Description=Favoritter - Self-hosted favorites web app
+After=network.target
+
+[Service]
+Type=simple
+User=favoritter
+Group=favoritter
+EnvironmentFile=/etc/favoritter/favoritter.env
+ExecStart=/usr/bin/favoritter
+Restart=on-failure
+RestartSec=5
+
+# Hardening
+NoNewPrivileges=yes
+ProtectSystem=strict
+ProtectHome=yes
+ReadWritePaths=/var/lib/favoritter
+PrivateTmp=yes
+ProtectKernelTunables=yes
+ProtectControlGroups=yes
+RestrictSUIDSGID=yes
+
+[Install]
+WantedBy=multi-user.target